JavaBeans lecture
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Transcript JavaBeans lecture
Object-Oriented Enterprise
Application Development
Introduction to JavaBeans
Topics
JavaBeans
Introduction
• What are they?
• Why should you care?
Merging with servlets
• Benefits
Merging with JSPs
• Benefits
• Techniques
This will not be comprehensive!
What are JavaBeans?
A Java Bean is a reusable software component
that can be manipulated in a builder tool.
JavaBeans is Java’s component model.
Components are self-contained units that can be
assembled to form complex systems.
These components must interoperate according to a
specified set of rules and guidelines.
JavaBeans provides the core concepts behind
Enterprise JavaBeans (EJBs).
JavaBean Concepts
Each component is called a bean.
A bean’s capabilities can be discovered at run time
without the developer knowing anything about the
bean ahead of time.
Like other objects, beans use their interface to
allow their state to be changed during run time.
Bean generally provide some mechanism for
saving and retrieving their internal state.
JavaBean Concepts (cont.)
To work effectively with beans, we need some
basic concepts.
Properties. The data manipulated by the bean. The
bean’s internal state.
Methods. The methods representing the bean’s
interface.
Events. The mechanism by which one bean sends
notifications to another. This is commonly done
through the user of callbacks.
JavaBean Concepts (cont.)
Beans are generally used at run time. Therefore
there must be a mechanism by which a client of
the bean can learn about that bean.
That process is called introspection.
Introspection is the process of exposing a bean’s
properties, methods, and events.
Instead of introspection, we can also provide an
explicit BeanInfo class (discussed later).
JavaBean Concepts (cont.)
Beans are often asked to save their state to
some non-volatile medium such as a disk or
database.
This is known as persistence.
Each JavaBean must provide some sort of
persistence mechanism.
One approach is to use Java Object Serialization by
implementing the java.io.Serializable
interface.
This is not the only approach.
PhraseBean.java
The bean implemented on the following page is
a very simple JavaBean.
It contains a single property, three methods,
and no events.
The single property is both readable and writable.
It uses Java Object Serialization.
PhraseBean.java
public class PhraseBean
implements java.io.Serializable {
private String phrase;
public PhraseBean() {
phrase = “Hello, World!”;
}
public void setPhrase(String phrase) {
this.phrase = phrase;
}
public String getPhrase() {
return (phrase);
}
}
PhraseBean
This is a legitimate JavaBean.
JavaBeans can implement the
java.io.Serializable interface if they
require persistence.
This allows the bean to be persisted if necessary.
This isn’t the only persistence mechanism available.
All beans should have a default constructor.
This allows us to create them using the
instantiate() method (discussed later).
Using the PhraseBean
A logical place to use this kind of bean is a JSP.
Since a bean is just a Java class, it can be used
anywhere that a Java class can be used.
Accessing a bean within a JSP simply requires a bit
more work.
To use the bean we must first create it and then
trigger one of its accessor methods.
Once a bean has been created we can use its
properties and methods.
Phrase.jsp
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0
Transitional//EN">
<HTML>
<HEAD><TITLE>Phrase JSP</TITLE></HEAD>
<BODY>
<jsp:useBean id="phrase" class="se452.PhraseBean" />
<H2>
<P>Old phrase is
<jsp:getProperty name="phrase"
property="phrase" />
</P>
<jsp:setProperty name="phrase"
property="phrase”
value = “Hello again, World!” />
<P>New phrase is
<jsp:getProperty name="phrase"
property="phrase" />
</P>
</H2>
</BODY></HTML>
useBean Tag
The useBean tag creates a new instance of the
bean specified by the class attribute and
associates it with the name provided by the id
attribute.
The example in the code creates a new instance of
the se452.PhraseBean class and assigns it to
a variable called phrase:
<jsp:useBean id="phrase" class="se452.PhraseBean"/>
getProperty Tag
The getProperty tag retrieves the property
specified by the property attribute from the
bean specified by the name attribute.
Before you can get a bean’s properties, that bean
must be created with the useBean tag.
The example in the code retrieves the current
value of the phrase bean’s phrase property:
<jsp:getProperty name="phrase" property="phrase"/>
getProperty Tag
Instead of using the getProperty tag:
<jsp:getProperty name="phrase" property="phrase"/>
we could have used a JSP expression:
<P>Old phrase is
<%= phrase.getPhrase() %>
</P>
Using the getProperty tag makes it clear
that we are using a bean and not another generic
object.
setProperty Tag
The setProperty tag allows us to change
the property specified by property for the
bean specified by name to the value specified
by value.
Before you can set a bean’s properties, that
bean must be created with the useBean tag.
The example in the code sets the current value
of the phrase bean’s phrase property:
<jsp:setProperty name="phrase"
property="phrase”
value = “Hello again, World!”/>
setProperty Tag
Instead of using the setProperty tag:
<jsp:setProperty name="phrase"
property="phrase”
value=“Hello again, World!” />
we could have used a JSP scriptlet:
<% phrase.setPhrase(“Hello again, World!”) %>
Using the setProperty tag makes it clear
that we are using a bean and not another generic
object.
Properties and Introspection
You may have noticed that the getProperty
and setProperty tags are referencing the
bean’s properties, which are nothing more than
private data to the bean’s class.
Behind the scenes however, the JSP is
converting property attributes to method
calls:
setProperty: changes phrase property to
setPhrase method call.
getProperty: changes phrase property to
getPhrase method call.
Linking Forms and Beans
Since beans are really where the program logic
for a JSP should reside, it makes sense that we
should have some mechanism for linking bean
properties to user input entered on an HTML
form.
One method might be the following:
<jsp:setProperty name=“bean” property=“beanprop”
value = “<% request.getParameter(“formparam”) %>”
/>
This approach is cumbersome at best although
it is also extremely flexible.
Linking Forms and Beans
Because we want to link forms and beans so
often, there are several shortcuts for doing so.
If the form element has the same name as a
bean property we can use the following syntax:
<jsp:setProperty name=“bean” property=“formparam”
/>
The JSP will look for a property on the bean
called formparam and will assume that the
value comes from a request parameter with the
same name.
Linking Forms and Beans
Often, the element names on a form don’t
match the properties on a bean.
In this case we can use the following variation
of the setProperty tag:
<jsp:setProperty name=“bean” property=“beanprop”
param=“formparam” />
The JSP will look for a request parameter called
formparam and will assign it to the bean’s
beanprop property.
Linking Forms and Beans
The last method of linking forms to bean
properties asks the JSP engine to search through
all the request parameters and to link them to the
corresponding bean properties.
In this case we can use the following variation
of the setProperty tag:
<jsp:setProperty name=“bean” property=“*” />
This leaves too much to chance, particularly if
the elements on the form, for whatever reason,
don’t match to the properties on the bean.
Serialization
To be considered as a bean, a class must
provide a means of serializing itself.
This allows the bean’s state to be serialized, or
persisted. even though the JVM in which the
bean executes shuts down.
The most common way of performing this
serialization is to have the bean implement the
java.io.Serializable interface.
There are other ways of providing this persistence.
Serialization (cont.)
The bean saves itself to a special file called a
serial file. All of its properties will be saved to
this file.
Each property to be saved must also be serializable.
If a given property should not be serialized,
then it needs to be marked as transient.
Properties that maintain state-specific data such as
socket or database connections shouldn’t be
serialized.
We can’t guarantee that when they’re re-hydrated,
that their state will make sense.
Using a Serialized Bean
Using a serialized bean is similar to using a
standard bean. There is a slight change to the
useBean tag:
<jsp:useBean id=”phrase” beanName=“phrase”
type="se452.PhraseBean"/>
The beanName attribute identifies the file in
which the serialized bean is stored.
By default an extension of .ser is used for
persisted beans although you never explicitly state
this.
Configuring Tomcat
At present Jakarta-Tomcat doesn’t support the
use of serialized beans.
This is an active work-in-process with their
group.
Bean Scope
By default beans only have scope local to the
page on which they are created.
This means that, by default, it isn’t possible for
a bean on one page to change a value in a bean
on another page and have that value appear on
that page.
There are ways to change this default behavior
by changing the bean’s scope.
Page Scope
The default scope for beans is page scope. A
bean with page scope can be created by using a
slightly modified useBean tag that includes
the scope attribute:
<jsp:useBean id="phrase" class="se452.PhraseBean”
scope=“page”/>
Each time the page is requested, the bean will
be created, be manipulated by the JSP code, and
disappear when the JSP is done executing.
Request Scope
We can create our beans so that they last as long
as the request object for the JSP exists:
<jsp:useBean id="phrase" class="se452.PhraseBean”
scope=“request”/>
This means that the bean will be available even
if the request is sent to another JSP using the
forward() or include() methods on the
RequestDispatcher object.
Session Scope
It is common to want to store information so
that whenever a user visits some location of the
site, their information is available.
In servlets we used the HttpSession object
to capture this data.
To make beans available this way, we use the
session scope:
<jsp:useBean id="phrase" class="se452.PhraseBean”
scope=“session”/>
Session Scope
When dealing with session scope, there are
sometimes cases where we only want a bean to
perform some action when it is first created.
For instance we might want to capture some
initialization data based on the request object such as
the user’s browser configuration.
For page and request scopes we always get
a new bean so this behavior makes no sense, but
for session scope we need to control this
behavior.
useBean Tag Revisited
We can configure our JSP to only perform certain
bean initialization code when the bean is first
constructed.
This involves the use of an end tag for useBean
start tag.
Phrase.jsp
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML>
<HEAD><TITLE>Phrase JSP</TITLE></HEAD>
<BODY>
<jsp:useBean id="phrase" class="se452.PhraseBean”
scope=“session”>
<jsp:setProperty name=“phrase” property=“phrase”
value=“Hello first time, World!” />
</jsp:useBean>
<H2>
<P>Old phrase is
<jsp:getProperty name="phrase" property="phrase" />
</P>
<jsp:setProperty name="phrase" property="phrase”
value = “Hello again, World!” />
<P>New phrase is
<jsp:getProperty name="phrase" property="phrase" />
</P>
</H2>
</BODY></HTML>
useBean Tag Revisited
Any JSP code, including HTML statements,
declarations, scriptlets, and bean accessors or
modifiers, will only be executed when the
specified bean is first created.
Since this is based on the user’s session, each user
will get this first-time behavior no matter how
many user’s have visited the page before them.
Application Scope
Sometimes we want data to be visible to all
users and across multiple pages.
Maintaining multiple beans that all store the
same data isn’t efficient in terms of resources.
To make beans available across the entire
application, irrespective of user or page, we use
the application scope:
<jsp:useBean id="phrase" class="se452.PhraseBean”
scope=“application”/>
Typecasting
All of the data sent to the JSP via the request
object is String data.
If a bean property expects data other than String
data, the JSP engine will attempt to perform an
explicit typecast using pre-defined conversion
rules.
These typecasts could fail.
Strings containing letters cannot be successfully
converted to integers.
Typecasting
If a conversion fails, then the mutator method
will not be called but an exception will not
necessarily be raised.
Obviously this is not desirable; our bean could
be in an inconsistent state which could easily
cause runtime problems later on.
One solution is to use indicator variables within
the bean to determine if a property was set
successfully.
CountBean.java
import java.io.*;
public class CountBean
implements java.io.Serializable {
private int count;
private boolean countOk;
public CountBean() {
count = 0;
countOk = true;
}
CountBean.java
public void setCount(String count) {
try {
this.count = Integer.parseInt(count);
this.countOk = true;
}
catch (Exception exc) {
this.countOk = false;
}
}
public int getCount() {
return (count);
}
CountBean.java
public boolean isValid() {
return (countOk);
}
public String getReason() {
if (!countOk) {
return (“Last.attempt.to.set.count.failed.”);
}
else {
return (“”);
}
}
}
Count.jsp
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0
Transitional//EN">
<HTML>
<HEAD><TITLE>Count JSP</TITLE></HEAD>
<BODY>
<jsp:useBean id="countBean" class="se452.CountBean"
scope="session" />
<jsp:setProperty name="countBean" property="count"
param= "countfield" />
<%
String text = null;
if (!countBean.isValid())
text = countBean.getReason();
else
text = "new count is " + countBean.getCount();
%>
<H2><%= text %></H2>
</BODY></HTML>
Events
Originally beans were designed to be graphical
components such as buttons, slider bars, and
lists.
This lead to the development of an event model
where these classes could listen for events in
which they were interested.
This is the same idea as that behind most GUI
development involving AWT and Swing.
It turns out that the event model has relevance
to server side processing as well.
Event Concepts
There are three key elements to the event
model:
Event objects. This encapsulates the information
that's specific to an instance of a given event. This
includes such information as the event's source.
Event sources. The object that triggered, or fired,
the event in the first place.
Event listeners. An object that is notified when a
specific type of event has been fired.
Interaction
The event model works as show in this diagram:
Event
Source
Register Event Listener
1. An event listener registers itself with the event
source, specifying the events that it should be
notified about.
Event
Object
Fire Event
2. When appropriate, the event source will notify
all listeners that a particular event has fired. Only
those listeners registered for that type of event
will be so notified.
Event
Listener
Events in Java
The java.util package contains the basic
classes used by the event model.
java.util.EventObject
java.util.EventListener
There is no specific class for an event source.
Instead, when an object wishes to trigger an
event, it invokes a callback method on each
listener that is interested in that event.
EventObject Class
All events extend
java.util.EventObject.
This class contains important information such as:
• A reference to the event source.
It also provides essential capabilities such as:
• Retrieving the event source.
• Providing itself as a human-readable string.
By convention, all of your event subclasses
should end in the word Event.
PhraseChangedEvent Class
Suppose that I want to be able to notify listeners
that the PhraseBean's phrase has been changed.
To enable this, I first create a new event class
called PhraseChangedEvent.
This class need not provide any new
functionality, so all its going to do is provide a
single-argument constructor.
PhraseChangedEvent.java
import java.util.*;
public class PhraseChangedEvent
extends java.util.EventObject
{
PhraseChangedEvent(Object eventSource)
{
super(eventSource);
}
}
Listeners
A listener is simply an object that is notified
whenever one of a set of events occurs in which
that listener is interested.
This is the basic principle behind all web,
application, and database servers.
When a request comes in on the appropriate port, a
web server knows that it has work to do.
PhraseChangeListener Interface
The new PhraseChangeEvent object is
fairly simple and doesn't provide much
functionality.
Nevertheless, it is sufficient to allow a
PhraseBean object to notify any listeners
that it might have that the phrase has changed.
The next step is to define an interface to be
implemented by objects that listen for these
notifications.
This will be the PhraseChangeListener
interface.
PhraseChangedListener.java
import java.util.*;
public interface PhraseChangedListener
extends java.util.EventListener
{
public void phraseChanged(PhraseChangedEvent
event);
}
EventListener Class
The java.util.EventListener interface
contains no methods. It is simply used to
indicate that a given interface is used to listen
for events.
It is a good idea to extend it into your own
interface instead of implementing it directly.
This allows for a hierarchy of interfaces to be
defined. Each interface provides only those method
signatures in which it is interested.
Listeners can then implement only the interfaces
they need.
PhraseListener Class
Now that we have a listener interface, we can
implement a concrete listener.
This will be the PhraseListener class.
Its job is to register itself with a phrase bean and be
notified when a phrase changes.
PhraseListener.java
import java.util.*;
public class PhraseListener
implements PhraseChangedListener
{
public void
phraseChanged(PhraseChangedEvent event)
{
System.out.println("Phrase.bean.has.changed");
}
}
Event Sources
There are no special classes to extend or
interfaces to implement for an event source.
It does need to provide the ability for event
listeners to register and de-register themselves.
This is done by providing two key methods:
public void add<ListenerType>(<ListenerType listener);
• This registers the listener with the event source.
public void remove<ListenerType>(<ListenerType
listener);
• This de-registers the listener with the event source.
PhraseBean Revisited
In order to allow the PhraseBean class to act as
an event source, we need to implement the
following two methods:
public void addPhraseChangedListener(
PhraseChangedListener listener);
public void removePhraseChangedListener(
PhraseChangedListener listener);
Note that the method names are by convention
only. However, following this convention will
ensure compatibility with all third-party tools
used to assemble beans.
PhraseBean.java
import java.io.*;
import java.util.*;
public class PhraseBean
implements java.io.Serializable {
private String phrase = null;
private Vector phraseChangeListeners = null;
public PhraseBean() {
phrase = "Hello, World!";
phraseChangeListeners = new Vector();
}
public void setPhrase(String phrase) {
this.phrase = phrase;
notifyPhraseChange();
}
PhraseBean.java (cont.)
public String getPhrase() {
return (phrase);
}
public synchronized void
addPhraseListener(PhraseListener listener) {
if (!phraseChangeListeners.contains(listener)) {
phraseChangeListeners.addElement(listener);
}
}
public synchronized void
removePhraseListener(PhraseListener listener) {
if (phraseChangeListeners.contains(listener)) {
phraseChangeListeners.removeElement(listener);
}
}
PhraseBean.java (cont.)
protected void notifyPhraseChange() {
PhraseChangedEvent event = new
PhraseChangedEvent(this);
Vector vec;
synchronized(this) {
vec = (Vector)phraseChangeListeners.clone();
}
int count = vec.size();
for (int i = 0; i < count; i++) {
PhraseListener client =
(PhraseListener)vec.elementAt(i);
client.phraseChanged(event);
}
}
}
PhraseListener Revisited
Now that the PhraseBean can support the
notification of each of its listeners, we can
allow the PhraseListener class to register
itself with the PhraseBean.
The basic change is that when a listener is
created it is given a reference to a bean. This is
what allows the listener to be notified when the
bean changes.
PhraseListener.java
import java.util.*;
public class PhraseListener
implements PhraseChangedListener {
private PhraseBean phraseBean;
public PhraseListener(PhraseBean phraseBean) {
this.phraseBean = phraseBean;
phraseBean.addPhraseListener(this);
}
public void phraseChanged(PhraseChangedEvent
event) {
System.out.println("Phrase.bean.has.changed");
}
}
Testing the Events
Testing the events consists of creating a bean
and then using it as the event source for one or
more event listeners.
The next example show a single bean acting as
the event source for multiple listeners.
TestPhrase.java
public class TestPhrase {
public static void main(String [] args) {
PhraseBean bean = new PhraseBean();
PhraseListener listener =
new PhraseListener(bean);
PhraseListener listener2 =
new PhraseListener(bean);
bean.setPhrase("this.is.a.new.phrase");
}
}
Review
During this class we have:
Seen how to construct and use basic JavaBeans.
Seen how to integrate JavaBeans components into JSPs.
Seen how to write events that allow one bean to trigger
actions on another using the JavaBeans event model.
References
Developing JavaBeans, Robert Englander,
O’Reilly and Associates, 1997.
JavaBeans, Part 1, Introducing JavaBeans, Greg
Voss, Sun Microsystems Whitepaper, November,
1996.
Coming Attractions
Next week we will:
Complete our discussion of JavaBeans.
• Study different kinds of properties such as indexed, bound, and
constrained.
• Discuss approaches to enhance the use of adapters and events.
• Further discuss the persistence mechanisms.
• Discuss the BeanInfo class as an alternative to introspection.